1. Field of the Invention
The present invention generally relates to an ion implanter, and more particularly, relates to an ion implanter having a variable aperture capable of flexibly shaping an ion beam before a wafer is implanted.
2. Description of the Prior Art
FIG. 1A is a sectional view for a conventional implanter 100. The conventional implanter 100 has at least an ion source 110, a mass analyzer 120, and a holder (not shown). The ion source 110 is capable of providing an ion beam 10. The mass analyzer 120 is capable of analyzing the ion beam 10 so that the ions with un-desired charge-to-mass ratios are moved away from the ion beam 10. After that, the ion beam 10 with required ions is implanted into the wafer 15 held by the holder.
As usual, the ion beam 10 outputted from the mass analyzer 120 is not as perfect as required. For example, the shape (the cross-sectional shape) of the ion beam 10 may be irregular or the ion beam current distribution among/along the cross-section of the ion beam 10 may be non-uniform. Therefore, an optimal or popularly required ion beam is not properly provided. This is true for both types of beams typically required, the spot ion beam and the ribbon ion beam.
Some prior art approaches, such as shown in FIG. 1B, improve on the disadvantage by applying at least one magnetic field between the mass analyzer 120 and the holder (or the wafer 15), such that the motion trajectories of ions of the ion beam 10 are adjusted by the interaction between the ions and the magnetic field. For example, the first bar magnet 131 and the second bar magnet 132 can be used to apply the magnetic field on the ion beam 10 before the wafer 15 is implanted. Reasonably, the ion beam 10 may be effectively shaped by properly adjusting the magnetic field. Different portions of the original ion beam 10 may have different deformations induced by the applied magnetic field, and the ion beam current distribution of the shaped ion beam 10 may be changed.
However, the method for adjusting the ion beam 10 by applying a magnetic field is difficult and complicated. The adjusted result is improved when the applied number of magnetic fields is increased. However, the cost and the difficulties also are correspondingly increased. Moreover, different portions of the ion beam 10 almost cannot be separately adjusted, because the magnetic field provided by a given magnet will be continuously distributed over the space.
Another prior art approach, such as shown in FIG. 1C, improves on the disadvantage by locating a plate 133 having a fixed aperture 134 between the mass analyzer 120 and the holder (or the wafer 15), such that only a portion of the ion beam 10 can pass through the fixed aperture 134 and be implanted into the wafer 15. Reasonably, when the plate 133 with fixed aperture 134 is properly selected to have a specific shape/size of the fixed aperture 134, the shape of the implanted ion beam 10 may be suitably changed. Moreover, by adjusting the relative geometric relation between the fixed aperture 134 and the ion beam 10, only a specific portion of the ion beam 10 with a specific ion beam current distribution can be implanted into the wafer 15.
However, the plate 133 with the fixed aperture 134 can only passively select a portion of the ion beam 10, especially when selection of a portion with a specific shape and size is needed. Hence, the potential improvement of ion beam 10 using this method may be limited. Moreover, when the required adjustments of the ion beam vary over a wide range, numerous plates 133 with different fixed apertures 134 are needed to provide the required adjustment freedom. Hence, the associated high cost of numerous plates 133 with different fixed apertures 134 and complex operations for replacing different plates 133 become unavoidable.
Because of disadvantages associated with prior art approaches as mentioned above, there is a need to find a novel ion implanter and a novel method for adjusting the ion beam effectively and economically.